The invention relates to a method of forming complex structures and stiffened panel structures, and particularly to an accordion expansion process.
During the past forty years, sandwich and other complex structures have attained widespread use particularly in the aircraft industry in the wings, wall panels, webs of beams, and the like.
A novel forming process, entitled "Method of Making Sandwich Structures" is disclosed in U.S. application Ser. No. 158,845, by Leonardo Israeli, which is incorporated into this specification by reference. This method makes sandwich structures by utilizing two outer workpieces and a plurality of core workpieces that are initially stacked together with portions of the core workpieces being cut out and other portions covered with a stop-off material to prevent diffusion bonding. By applying pressure between the outer workpieces, the core workpieces are expanded to form vertical webs. One disadvantage of these vertical web structures is the lower core stability to normal and shear plate loads as compared to webbing forming oblique angles relative to the outer workpieces. Another disadvantage of the above method is that when the sandwich structure is formed into a chamber having a partially oblique surface, superplastic forming is needed, which requires superplastic materials heated to superplastic forming temperatures. This combining of superplastic forming with accordion expansion causes additional fabrication problems, since superplastic forming temperatures generally are much higher than those temperatures required for accordion expansion.
Superplastic forming properties are exhibited by only a small number of metals and alloys, and the process involves the capability of a material to develop unusually high tensile elongations and plastic deformation at higher temperatures with a reduced tendency toward thinning or necking. (See for example U.S. Pat. Nos. 3,934,441 and 4,181,000). In superplastic forming the workpiece is heated until it becomes superplastic, after which differential pressure is applied causing the workpiece to stretch and form into a cavity. In addition to being limited to a small number of metals and alloys, the excessive stretching may result in non-uniform strength and thickness of the formed structure. The forming process is a complex one with critical parameters (time, temperature, and pressure) controlling the rate of stretching. Necking and ruptures are the direct result of exceeding the narrow tolerances of those parameters.
Diffusion bonding, which is often combined with superplastic forming, is the metallurgical joining of surfaces by applying heat and pressure for a sufficient time as to cause commingling of the molecules at the joint interface. The basic requirement for diffusion bonding is to bring the clean mating surfaces close enough together to allow the inter-molecular attractive forces to become effective.